1. Field of the Invention
The present invention relates to a hold type image display system.
2. Description of the Related Art
Conventionally, a liquid crystal display (LCD) is generally being widely used since it is thin and does not occupy as much installation area compared to a CRT (Cathode Ray Tube) display. However, in the hold type display device such as LCD, an image continues through a frame period as opposed to an impulse type display device such as CRT display, and thus a moving image tends to become unclear.
In the case of the impulse type display device, an image is displayed as a pulse at an early stage in the frame, and a black display is displayed until the next frame, and thus the afterimage occurrence is adjusted not to be recognized by the user's eyes. In the case of the hold type display device, on the other hand, the image is held and displayed as a still image within the frame period, and the moving image is displayed by switching the screen for every frame, and thus the still image is seamlessly switched from one frame to another, whereby the user recognizes the frame image of one before as an afterimage, senses a double image in which the shifted images are overlapped and recognizes a moving image blur. In order to improve the moving image blur in the hold type display device, a hold type display device for pseudo-driving in an impulse type display is disclosed in Japanese Laid-Open Patent Publication No. 9-18814 (patent document 1).
However, the liquid crystal display device of patent document 1 switches between a black display and a video display for every ½ frame period of an input signal, and thus the black insertion rate with respect to one frame is limited to ½. Thus, in the device of patent document 1, setting an optimum black insertion rate corresponding to each usage state with considering the balance between improvement in moving image blur, which is a merit of black insertion, and lowering in luminance, which is a demerit of black insertion, cannot be realized.
There is a number of liquid crystal driving methods in accordance with the liquid crystal panel, and if the liquid crystal driving method differs in accordance with each type of panels such as TN type, IPS type, VA type, and OCB type, the response property differs and the optimum black insertion rate also differs. With the device of patent document 1, the application of the black insertion drive suited for all the methods was difficult. Furthermore, in the liquid crystal display device of patent document 1, the circuit scale increases and also the cost increases since the drive frequency of the device is required to be doubled and a frame memory for temporarily storing video information becomes necessary.
On the other hand, a device which increases the degree of flexibility of black insertion rate and avoids increase in circuit scale is disclosed in Japanese Laid-Open Patent Publication No. 2001-166280 (patent document 2). As shown in FIGS. 14 to 16, the liquid crystal display device of patent document 2 has a configuration of individually inputting a start pulse signal (VSP) to each gate driver, inputting an enable signal (VOE), or a control signal, to half of the plurality of gate drivers, and inputting an enable signal whose polarity is inverted by an inverter to the remaining half of the gate drivers. One gate line is selected from each half of the gate drivers, respectively, where an video signal corresponding to the enable signal is written to the pixel on one gate line, and a black signal corresponding to the enable signal which polarity is inverted is written to the pixel on the other gate line, so that the black insertion rate of gate driver segmentation can be changed.
However, when commercializing a product having a resolution used for TV, if the gate driver offered in the market is used, two gate drivers are generally used in the VA (Video Graphics Array), and three gate drivers are used in the XGA (Extended Graphics Array) and WXGA (Wide XGA), whereas, in the configuration of the device of patent document 1, the gate driver is limited to an even number, and thus the degrees of flexibility in selecting the number of gate drivers lower when it is applied to a product, and in some cases, an extra gate driver IC becomes necessary thereby a wasteful cost is required.
In the device of patent document 2, one gate driver starts the scanning of the video display, and at the same time, another gate driver specified in advance starts the scanning of the black display, and thus the black insertion rate can only be set at the segment of the gate driver and operation is performed with the fixed black insertion rate with respect to one frame, whereby it is difficult to set an optimum black insertion rate variably according to each usage state.
Furthermore, in the device disclosed in patent document 1, the continuous holding in a blanking period is not taken into consideration in the case of writing the black image over a plurality of frames. Thus, as shown in FIG. 15, a time interval corresponding to the blanking period is created when the writing of the black signal runs to the next frame, thereby creating a difference in the black image holding time between the upper half and the lower half of the screen, and a luminance difference as shown in FIG. 16 is displayed in the one screen, with the line from where the difference in the black image holding time is created as a boundary.